Instead of scale and whatnot, pass:
1. The image size
2. The viewport to map to that image size
and compute everything else from there.
In particular, we set the Vulkan viewport to the image dimensions
instead of the viewport size.
All of this makes things a lot simpler while keeping the required
functionality.
Add an explicit begin() and an end() op. For now, this looks like
overkill, but it allows doing renderpasses with custom ops that are not
meant to render a rendernode.
Examples for this are pre/postprocessing passes or 2-pass blur.
The API was using regions because it always had. But all the code ever
did was get the extents of the region.
So simplify everything by using rectangles everywhere.
These days, we can query it with gsk_vulkan_render_get_context().
Makes quite a few functions require one less argument.
And it also makes the GskVulkanRenderPass empty. Gotta figure out what
to do with it.
The render pass ops were not updating the image's layout to the final
layout when a render pass ends.
Fix that.
Also make the layouts explicit arguments to the render pass op.
... instead of doing the equivalent things manually by creating a
RenderPass and calling the relevant functions.
Now all renderpass operations are indeed stored in ops.
Also reshuffle the command emission code, because we no longer need to
emit the ops for the base renderpass.
As a result we only submit a single command buffer containing all the
render passes instead of once per render pass.
We also bind vertex buffers and descriptor sets only once now at the
start instead of once per renderpass.
